The invention provides a video image-based method and a video image-based system for detecting indoor smog. The method for detecting the indoor smog comprises the following steps of: acquiring the video image through an ordinary monitoring camera; initially detecting the smog and acquiring an initial smog detection region in a current frame image by analyzing the gradient and gray scale information of each frame image; and accurately detecting the smog, filtering a false region in the initial smog detection region and acquiring an accurate smog detection region for a video. Compared with the prior art, the method and the system have high sensitivity for the indoor smog and strong anti-interference and realize indoor video smog detection in a large space.